One of the strongest earthquakes in recent history ( 8.9 Mw) has struck Japan some hours ago, and surely you are well informed by now. In any case, you can find up to date information in the following websites:
The tsunami alert has been extended to most of the Pacific, where people are strongly advise not to stay along the coasts.
The 03/11/2011 earthquake (preliminary magnitude 8.9) near the east coast of Honshu, Japan, occurred as a result of thrust faulting on or near the subduction zone interface plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a velocity of 83 mm/yr. The Pacific plate thrusts underneath Japan at the Japan Trench, and dips to the west beneath Eurasia. The location, depth, and focal mechanism of the March 11 earthquake are consistent with the event having occurred as thrust faulting associated with subduction along this plate boundary. Note that some authors divide this region into several microplates that together define the relative motions between the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are respectively part of North America and Eurasia.
The March 11 earthquake was preceded by a series of large foreshocks over the previous two days, beginning on March 9th with an M 7.2 event approximately 40 km from the March 11 earthquake, and continuing with a further 3 earthquakes greater than M 6 on the same day.
The Japan Trench subduction zone has hosted 9 events of magnitude 7 or greater since 1973. The largest of these was an M 7.8 earthquake approximately 230 km to the north of the March 11 event, in December 1994, which caused 3 fatalities and almost 700 injuries. In June of 1978, an M 7.7 earthquake 75 km to the southwest caused 22 fatalities and over 400 injuries. In December of 2008, a sequence of 4 moderate earthquakes (M 5.3-5.8) occurred within 20 km of the March 11 event. In the first 12 hours following the March 11 earthquake, the region has experienced over a dozen aftershocks of M 5 or greater, the largest being M 5.7.
Scientific data about the earthquake, from USGS
Epicenter: 38.321 142.369
USGS/WPHASE CENTROID MOMENT TENSOR
Centroid: 37.321 141.769
Depth 24 No. of sta: 89
Moment Tensor; Scale 10**22 Nm
Mrr= 1.16 Mtt= 0.18
Mpp=-1.34 Mrt= 1.33
Mrp= 2.17 Mtp=-0.10
T Val= 2.88 Plg=55 Azm=313
N = -0.08 11 205
P = -2.79 32 108
Best Double Couple:Mo=2.8*10**22
NP1:Strike=162 Dip=17 Slip= 45
NP2: 28 78 102